1. Field of the Invention
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming capable of effectively correcting alignment errors of elementary color outputs to be superposed into an image.
2. Description of the Background Art
An image forming apparatus forming a color image is generally known. In such an image forming apparatus, a plurality of elementary-color images, that is, latent images, in response to a color image having respective color components are formed on photoconductors by using a plurality of image writing mechanisms. Then, the latent images with the respective color components formed on the photoconductors are developed by color toners, so that color toner images are formed on the respective photoconductors. Then, the color toner images thus formed are superposed and transferred onto a recording medium. Finally, a single color image is formed on the recording medium.
The image forming apparatus described above includes a pattern forming mechanism configured to form patterns with respective color toners, on the photoconductors, used for detecting positional displacements, during an interval of image forming processing on the recording medium, a detection mechanism configured to detect an amount of positional displacements in writing positions of the respective color components, based on positions of toner patterns formed on the respective photoconductors by the pattern forming mechanism, and a correction control mechanism configured to roughly correct the positional displacements in the writing positions with respective color components during a first interval and correct remaining positional displacements in plural times during a second interval or later intervals, based on the amount of the positional displacements in the writing positions of the respective components detected by the detection mechanism.
In such an image forming apparatus described in one document associated with background art, sub-scan color displacements, as an object to be corrected, can be corrected in timing with a between-sheet interval. However, the document does not describe a method for correcting main-scan color displacements, in detail. The document schematically describes that the same method can be applied thereto.
Causes, an amount, and directions of errors to be corrected may be different between correction in the main scanning direction and in the sub scanning direction. Therefore, various countermeasures may be taken to correct the errors.
For example, positional displacements in the main scanning direction, that is to say, a color displacement, may result from a positional variation with time in scan mechanisms, such as a polygon mirror, positional variation with time in optical members such a mirror, etc. In addition, positional displacements in the main scanning direction may also result from accumulated errors in the respective images formed independently. Thereby, positional displacements in a writing start point from a reference position cause positional displacement errors in the main scanning direction between respective colors, so that color displacements are caused. On the other hand, sub-scan color displacements based on a writing error results from a rotation velocity of a mechanism for conveying in the sub scanning direction.
Moreover, as for the errors in the main scanning direction, a magnification error is also known. Suppose a case in which a mold lens group is used for an optical system that guides scanning light to the photoconductor. Quite different from glass lenses unsusceptible to environmental conditions including temperatures, a mold lens group is susceptible to environmental conditions and operating conditions. Accordingly, a refractive index of the mold lens group tends to frequently vary. The magnification error is a certain type of error in which the magnification in the main scanning direction is varied by heat generated by a laser drive part and a polygon motor drive part. Therefore, the error is regarded as “the magnification error in the main scanning direction”, and the error is a primary reason for image distortion and color displacement.
An amount of the magnification error in the main scanning direction resulting from variation in refractive index is large. This is because the magnification error in the main scanning direction is caused by temperature variation with time from a point of time when the image forming apparatus is started up. However, since the magnification error in the main scanning direction does not require the patterns used for error detection, a time required for detecting the amount of correction and a time required for correction may be short. On the other hand, a required time period for the positional displacement errors in the main scanning direction and the sub scanning direction is relatively longer. This is because the patterns used for the error detection are formed in detecting the amount to be corrected.
As described above, there are several types of errors with respect to the positional displacements of the image formed on the medium holding the images thereon. In particular, in the image forming apparatus and an image forming method forming the superposed images, since positional displacements are more apparent than color displacements, correcting the positional displacement errors is indispensable to improve image quality. Whereas on the other hand, upon correcting such errors to shorten processing time for forming the image, decrease in processing capability for forming the image needs to be avoided.